1. Field of the Invention
The present invention relates to an active matrix type display device for making display of a large screen with high resolution.
2. Description of the Related Art
In recent years, a technique for manufacturing a semiconductor device in which a semiconductor film is formed on an inexpensive glass substrate, such as a thin film transistor (TFT), has been rapidly developed. The reason is that demand for an active matrix type liquid crystal display device (liquid crystal panel) has been increased.
The active matrix type liquid crystal panel is structured such that a TFT is disposed for each of several tens to several millions of pixel regions arranged in matrix, and an electric charge going in and out of the respective pixel electrodes is controlled by the switching function of the TFT.
FIG. 1 shows a conventional active matrix type liquid crystal display device. As shown in FIG. 1, the conventional active matrix type liquid crystal display device includes a source line side driver 101, a gate line side driver 102, a plurality of pixel TFTs 103 arranged in matrix, and a picture signal line 104.
The source line side driver and the gate line side driver respectively include a shift register, a buffer circuit and the like, and in recent years, they are monolithically formed on the same substrate as an active matrix circuit.
In the active matrix circuit, thin film transistors using amorphous silicon formed on a glass substrate are disposed.
There is also known a structure in which a thin film transistor is formed of a polycrystalline silicon film while quartz is used for a substrate. In this case, both a peripheral driving circuit and an active matrix circuit are constituted by thin film transistors formed on the quartz substrate.
There is also known a technique in which a thin film transistor using a crystalline silicon film is formed on a glass substrate by using a technique such as laser annealing. When this technique is used, an active matrix circuit and a peripheral driving circuit can be integrated on the glass substrate.
In the structure as shown in FIG. 1, a picture signal supplied to the picture signal line 104 is selected by a signal from the shift register circuit of the source line side driver (shift register for horizontal scanning). A predetermined picture signal is supplied to a corresponding source signal line.
The picture signal supplied to the source signal line is selected by a thin film transistor of a pixel and is written into a predetermined pixel electrode.
The thin film transistor of the pixel is operated by a selection signal supplied from the shift register of the gate line side driver (shift register for vertical scanning) through a gate signal line.
This operation is sequentially repeated at suitable timing by a signal from the shift register of the source line side driver and a signal from the shift register of the gate line side driver, so that information is sequentially written into each of the pixels arranged in matrix.
FIG. 2 is a schematic view showing the scan of the respective pixels by such a conventional driving method. Reference numeral 201 denotes a source line side driver, 202 denotes a gate line side driver, and 203 denotes a pixel region in which a plurality of pixel TFTs are arranged in matrix. Reference numeral 204 denotes the scanning direction of the pixel TFTs.
The pixel TFTs are sequentially scanned in the direction 204, and after image information for one picture is written, writing of image information for a next picture is carried out. In this way, the display of picture images is sequentially carried out. In general, the writing of information for one picture is carried out 30 times or 60 times a second.
In recent years, with the rapid increase of the amount of information to be treated, it has been designed to increase the display capacity and to improve display resolution. Here, examples of generally used display resolution of a computer will be shown below with the number of pixels and the name of standards.
Number of pixels:(Horizontal × Vertical)Name of standards640 × 400EGA640 × 480VGA800 × 600SVGA1024 × 768 XGA1280 × 1024SXGA
Recently, also in the field of a personal computer, since software for making a plurality of expressions with different characters on a display has come into wide use, a display device corresponding to the XGA or SXGA standard with resolution higher than the VGA or SVGA standard becomes common.
However, a display system according to a conventional structure has a problem described below in realizing the foregoing high display resolution.
In a conventional liquid crystal electrooptical device, due to such reasons that (1) the mobility of a thin film transistor is low, and (2) a long time is required for writing of data into liquid crystal pixels, a sampling clock frequency especially in a horizontal direction can not be increased, so that a high speed operation has been difficult.
Especially, as a displayed picture becomes large (the number of display pixels increases), these phenomena have been remarkable, since a large amount of data is used.
Moreover, the above-mentioned liquid crystal display device having high resolution comes to be used also as display of a television signal other than display of a data signal in a personal computer.
In recent years, in order to realize a beautiful picture quality as in a high definition TV (HDTV) or an extended definition TV (EDTV), image data for one picture becomes several times that of a conventional TV. Moreover, since the easiness of viewing is improved and it becomes possible to display a plurality of pictures on one display device by enlargement of a screen, a large screen comes to be increasingly required. For realization of these as well, it is urgently required to realize a high speed operation of a liquid crystal display device.